1. Field of the Invention
The present invention relates generally to the field of video amplifiers and more particularly to wide band, high power video amplifiers, which may be of the hybrid type, configured for providing fast rise time output signals to video displays.
2. Discussion of the Background
Video amplifiers are used to provide high voltage, amplified signals to video displays which typically present a relatively high capacitive load to the amplifiers. Present video amplifiers, having bandwidths of about 50 to 100 MHz, typically provide output signals having voltages of about 25 to 40 volts peak-to-peak, into loads of about 15 to 25 picofarads (pF). Under such conditions, the rise times of the output signals provided by the video amplifiers are typically in the range of about 4 to 8 nanoseconds.
Needs, however, presently exist for improved video amplifiers which meet the performance requirements of new generation, high resolution color, 1000 and 2000 line raster video displays, which have come into demand in military graphics, air traffic control displays and commercial CAD/CAM systems. These new generation video displays typically require extremely fast amplified signal rise times of only about 1 to 3 nanoseconds at 45 to 55 volts peak-to-peak, when driving into loads of between about 4 pf to about 15 pF. Amplifier bandwidths in excess of about 100 MHz are also typically required by such new generation displays.
The rise times of many currently available video amplifiers are thus seen to be comparatively slow with respect to the rise time requirements of new generation displays and result primarily from the relatively high capacitive load into which the amplifiers are required to operate. These high capacitive loads are typically comprised not only of the display capacitance but also, for example, of the distributed wiring, interconnect and circuit card capacitances, part-to-part capacitance and other parasitic capacitances associated with the electronic circuitry in the amplifier output network.
It can be appreciated that with the comparatively slow output signal rise times provided by typical, presently available video amplifiers, very short duration amplifier input signals, having pulse widths equal to or less than the amplifier output signal rise times, are "lost" and cannot be applied to the associated display. Some signal information is accordingly lost and the resolution of the associated display is less than would be possible if the output signal rise time were faster. Such loss of short-duration signals can adversely affect the performance of the display equipment and is, in any event, generally inconsistent with the requirements of most high resolution video displays.
Most video amplifiers presently use conventional solid state cascode amplifiers--that is, amplifiers having a common emitter-common base pair of transistors--as is more particularly discussed below in the description of the present invention. Variations, known as frequency compensation circuits, have been added to conventional cascode amplifiers to decrease the rise and fall times associated therewith. For example, the addition of a capacitance in the emitter leg of the common emitter transistor of a cascode amplifier, often called "emitter compensation," serves to increase the high-frequency current generated in the amplifier. When video pulses are amplified, this technique serves to square off the output signal and reduce the associated rise and fall times thereof. Another such technique is the addition of output inducatance in series and/or shunt with the load capacitance.
There are, however, characteristic problems with compensation circuits. For example, such circuits can, while, improving the rise time, cause a ringing in the output signal at turn on and turn off. Such signal ringing manifests itself in the display as repetitive "ghost" video (such as, for instance, dim vertical streaks following a displayed vertical line), which diminishes the sharpness of the video display and can cause operator confusion when the display is crowded with information. Consequently, additional circuitry is often needed to reduce the ringing caused by the compensation. As a result, wide bandwidth operation is sometimes not achieved.
From the foregoing, it can be appreciated that the development of new means for amplifier compensation is needed, which, when either used alone or combined with existing techniques, can achieve the high amplifier speeds presently coming into demand in today's new high resolution displays. The invention described herein implements a new such approach to amplifier compensation.